This invention relates generally to an electric fence and more particularly to controlling the output energy of one or more energisers which are used to electrify the fence.
Electric fences are in widespread use inter alia for livestock control and for security purposes. It is desirable to have a high level of energy output by an energiser to make a fence excitation voltage less susceptible to electrical load conditions on a fence and, in respect of a long fence, to reduce the number of energisers. However the energy output level prevailing on the fence must be non-lethal and must comply with legislative provisions.
FIG. 1 of the accompanying drawings illustrates a typical prior art arrangement 10 wherein a capacitor 12 of capacitance C is charged by an external charging circuit to a voltage V. A switch 14, in the nature of a thyristor, is used, in a controlled manner, to discharge the capacitor through a primary wounding 16 of a transformer 18. A voltage is then induced in a secondary winding 20 which is applied to a fence.
The energy E stored in the capacitor 12 is given by the expression E=½CV2.
It is evident that the output energy can be regulated by controlling the voltage V or the magnitude of the capacitance C. In the latter case a number of capacitors can be used to achieve energy regulation.
Typically energy control is achieved by regulating the voltage V. The fence excitation voltage is measured during a firing pulse and then, according to a control algorithm, the voltage to which the capacitor is charged for a subsequent discharge pulse is determined.
Generally an energiser is characterised and regulated by its performance across a 500 ohms load. Under certain heavy load conditions, for example if a low resistance (less than 500 ohms) prevails between output terminals of an energiser, then the output energy may substantially exceed the level which would arise if the energiser had been loaded with a load of 500 ohms or higher resistance.
A problem with this type of situation is that there is always a minimum delay of one pulse in the adaption of the energiser output energy level applied to the fence. Thus, if a person touches a heavily loaded fence and the load condition then changes to a lighter load, a substantial amount of energy can be injected into the fence before the energy level is dropped, and this may prove to be lethal to such a person.
International application No. PCT/NZ99/00212 addresses the aforementioned problem by controlling the energiser output according to the rate of change of the electrical load, on the fence, detected by a sensor.
The present invention is concerned with an alternative approach to the regulation of the output energy of an energiser.